Systems and methods for optical modulation index calibration in a CATV network

ABSTRACT

Systems and methods for optical modulation index calibration in a CATV network.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119(e)to U.S. Provisional Application No. 62/043,793 filed on Aug. 29, 2014,U.S. Provisional Application No. 62/052,213, filed on Sep. 18, 2014,U.S. Provisional Application No. 61/984,303 filed on Apr. 25, 2014, andU.S. Provisional Application No. 61/982,089, filed on Apr. 21, 2014.

BACKGROUND

The present disclosure relates to systems and methods that processsignals over a cable transmission network.

Although Cable Television (CATV) networks originally delivered contentto subscribers over large distances using an exclusively RF transmissionsystem, modern CATV transmission systems have replaced much of the RFtransmission path with a more effective optical network, creating ahybrid transmission system where cable content originates and terminatesas RF signals over coaxial cables, but is converted to optical signalsfor transmission over the bulk of the intervening distance between thecontent provider and the subscriber. Specifically, CATV networks includea head end at the content provider for receiving RF signals representingmany channels of content. The head end receives the respective RFcontent signals, multiplexes them using an RF combining network,converts the combined RF signal to an optical signal (typically by usingthe RF signal to modulate a laser) and outputs the optical signal to afiber-optic network that communicates the signal to one or more nodes,each proximate a group of subscribers. The node then reverses theconversion process by de-multiplexing the received optical signal andconverting it back to an RF signal so that it can be received byviewers.

Cable television (CATV) networks have continuously evolved since firstbeing deployed as relatively simple systems that delivered videochannels one-way from a content provider. Early systems includedtransmitters that assigned a number of CATV channels to separatefrequency bands, each of approximately 6 MHz. Subsequent advancementspermitted limited return communication from the subscribers back to thecontent provider either through a dedicated, small low-frequency signalpropagated onto the coaxial network. Modern CATV networks, however,provide for not only a much greater number of channels of content, butalso provide data services (such as Internet access) that require muchgreater bandwidth to be assigned for both forward and return paths. Inthe specification, the drawings, and the claims, the terms “forwardpath” and “downstream” may be interchangeably used to refer to a pathfrom a head end to a node, a node to an end-user, or a head end to anend user. Conversely, the terms “return path” “reverse path” and“upstream” may be interchangeably used to refer to a path from an enduser to a node, a node to a head end, or an end user to a head end.

Recent improvements in CATV architectures that provide furtherimprovements in delivery of content include Fiber-to-the Premises (FTTP)architectures that replace the coaxial network between a node and asubscriber's home with a fiber-optic network. Such architectures arealso called Radio Frequency over Glass (RFoG) architectures. A keybenefit of RFoG is that it provides for faster connection speeds andmore bandwidth than current coaxial transmission paths are capable ofdelivering. For example, a single copper coaxial pair conductor cancarry six simultaneous phone calls, while a single fiber pair can carrymore than 2.5 million phone calls simultaneously. FTTP also allowsconsumers to bundle their communications services to receive telephone,video, audio, television, any other digital data products or servicessimultaneously.

One existing impairment of RFoG communication channels is Optical BeatInterference (OBI), which afflicts traditional RFoG networks. OBI occurswhen two or more reverse path transmitters are powered on, and are veryclose in wavelength to each other. OBI limits upstream traffic, but alsocan limit downstream traffic. Existing efforts at mitigating OBI havefocused on Optical Network Units (ONUs) at the customer premises, or onthe CMTS at the head end. For example, some attempts to mitigate OBImake the ONUs wavelength specific while other attempts create anRFoG-aware scheduler in the CMTS. Still others attempts have includedchanging ONU wavelengths on the fly. Due to the fundamental nature oflasers and DOCSIS traffic, none of the above techniques yieldsatisfactory results as wavelength collisions still occur or cost ishigh. Thus, it may be desirable in RFoG deployments to further reduce oreliminate OBI.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows an existing RFoG architecture.

FIG. 2 shows an improved RFoG architecture.

FIG. 3 compares capabilities of the architectures of FIGS. 1 and 2.

FIG. 4 shows an RFoG transmission path between a CMTS and a cable modem.

FIG. 5 shows an improved ONU that mitigates clipping.

FIG. 6 shows a second improved ONU that mitigates clipping.

FIG. 7 shows an ONU output spectrum having a rise time of 100 ns.

FIG. 8 shows an ONU output spectrum having a rise time of 1000 ns.

FIG. 9 shows a response time of an ONU to an RF signal.

FIG. 10 shows an ONU having a laser bias and RF amplifier gain control.

FIG. 11 shows the response time of an ONU with RF gain control inproportion to laser bias control.

FIG. 12 shows the response time of an ONU where the RF gain control isdelayed with respect to the laser bias control.

FIG. 13 shows an ONU having a separate amplifier gain and laser biascontrol.

FIG. 14 shows a transmission line receiver structure.

FIG. 15 shows a transmission line receiver connection to a biasedamplifier.

FIG. 16 shows a transmission line receiver with photocurrent detectionat the termination side.

FIG. 17 shows an active combiner with multiple inputs and optical burstmode operation.

FIG. 18 shows an active combiner with optical burst mode operationincluding amplifier bias control.

FIG. 19 shows an active combiner with OBM, laser bias, amplifier biasand gain control.

DETAILED DESCRIPTION

FIG. 1 shows an exemplary existing RFoG system 10, where a head end 12delivers content to an ONU 14 at a customer's premises through a node16. An RFoG topology includes an all-fiber service from the head end 12to a field node or optical network unit (ONU), which is typicallylocated at or near the user's premises. In the head end 12, a downstreamlaser sends a broadcast signal that is optically split multiple times.The optical network unit, or ONU, recovers the RF broadcast signal andpasses it into the subscriber's coax network.

The head end 12 typically includes a transmitter 18 that delivers adownstream signal to one or more 1×32 passive splitters 20 that includes32 output ports, each output port connected to a wavelength divisionmultiplexer (WDM) splitter 28 that delivers the downstream content overa fiber transmission segment 24 to the node 16, which in turn includesanother 1×32 splitter 22, where each output port of the splitter 22 isconnected via another fiber segment 26 to a particular ONU 14 at asubscriber's premises.

Optical networking units (ONUs) in an RFoG environment terminate thefiber connection at a subscriber-side interface and convert traffic fordelivery over the in-home network at the customer premises. Coaxialcable can be used to connect the ONUs of an RFoG network to one or moreuser devices, where the RFoG user devices can include cable modems,EMTAs, or set-top boxes, as with the user devices of an HFC network. Forexample, the ONU 14 may connect to set-top boxes, cable modems, orsimilar network elements via coaxial cable, and one or more of the cablemodems may connect to the subscriber's internal telephone wiring and/orto personal computers or similar devices via Ethernet or Wi-Ficonnections.

Those of ordinary skill in the art will appreciate that the foregoingarchitecture is illustrative only. For example, the number of ports ofthe splitters 20 and 22 may be changed, as desired. It should also beunderstood that the head end 12 may include more splitters 20, eachsplitter having outputs connected to a respective node so as to serve agreat number of subscribers.

Along the return path from the subscriber's ONU 14 to the head end 12,the splitter 22 operates as a combiner, i.e. up to 32 ONUs may deliverreturn path signals to the node 16, which combines them for upstreamtransmission along fiber length 24. Each of the signals from therespective ONU's 14 is then separated from other signals by the WDM 28to be received by a separate receiver 30 in the head end 12. The signalsfrom the respective receivers are then combined by a combiner 32 fortransmission to a Cable Modem Termination Service (CMTS) in the head end12. The signals are combined in the RF domain in the head end 12 by thecombiner 32, before being connected to the CMTS upstream port. Combinedwith the forward power limit on the fiber, the combined signals requireone forward fiber (L1 km) per group of 32 subscribers.

In the forward direction, the forward transmitter is provided to ahigher power multi-port amplifier that distributes power. For example,in the head end 12, the transmitter 18 provides output to an ErbiumDoped Fiber Amplifier (EDFA) 34 that internally distributes power overthe 32 outputs of the combiner 20, each output operated at a relativelyhigh power, e.g. approximately 18 decibel-milliwatts (dBm). The WDM 28typically passes 1550 nm light from the EDFA 34 in forward direction anddirects reverse light, typically at 1610 nm or perhaps 1310 nm in thereverse direction to the receivers 30. The WDM 28 may be connected to afiber of length L1 that feeds the splitter 22 in the node 16. Theoutputs of the splitter 22 are each provided to second fibers of lengthL2 that are respectively connected to ONUs 14 at the subscriber homes.Typically, L1+L2 may be up to 25 km. The ONUs 14 convert the forwardtransmitted light to RF signals for the in-home coaxial network. TheONUs 14 also receive RF signals from the in-home network and modulatethese signals onto a laser, operating at 1610 nm for example, and thelaser's output is sent upstream into the fiber L2. The upstream signalis combined with other upstream signals in the combiner 22 andtransmitted further upstream in the fiber L1. At the WDM 28 the upstreamsignals are directed towards the head end receivers 30.

The loss budget for 32 subscribers and 25 km of fiber requires onereceiver in the head end 12 for every group of 32 subscribers; given anupstream transmission power of 3 dBm, the receivers 30 and the WDM 28may typically operate at a power between −18 and −21 dBm, making a goodsignal to noise ratio challenging, such that band limited receivers areusually required for acceptable performance. Furthermore, the passiveoptical combiner 22 that combines multiple optical inputs to a singleoutput by definition creates OBI between these inputs, as describedearlier and will therefore create noise in the RF domain at the head endreceivers 30. Furthermore, a loss of around 24 dB must also be assumedin the forward path; for an EDFA output power of 18 dBm per port, thisprovides −6 dBm power to the receivers. This is sufficient foracceptable performance at the ONU to 1 GHz, provided low noise, highgain receivers are used.

The disclosed techniques for eliminating OBI is desirable, and thedisclosed manner for eliminating OBI as disclosed may enable highercapacity in the upstream and downstream. Further, the disclosed combinerand features of the combiner may enable RFoG coexistence alongsidetraditional HFC/D3.1 systems and future potential PON systems. Theelimination of OBI is critical in some systems to unlock the vastpotential of the optical fiber. Described in more detail herein areembodiments for an architecture that incorporates the disclosed opticalcombiner system.

FIG. 2 shows an improved system 100 for delivering CATV content to aplurality of subscribers over an RFoG network. The architecture shows ahead end 110 having a transmitter 112 and receiver 114 each connected toa WDM splitter 116 that outputs a signal to, and receives a signal from,a fiber link 118 of L1 km. The fiber link 118 is connected to an activesplitter/combiner unit 120. The splitter/combiner unit 120 maypreferably include a WDM 122 that separates forward path signals fromreverse path signals. The forward path signal from the WDM 122 isprovided to an EDFA 124 that outputs an amplified optical signal to anactive 1×32 splitter 126 that has 32 output ports, each to respectivesecond fiber links 128. At each port, the power level can be modest(e.g. in the 0-10 dBm range) but can also be high (e.g. in the 18 dBmrange).

In the reverse direction, the 1×32 port splitter 126 operates as anactive combiner 126, and includes, at each port, a WDM that directsupstream light to a detector at the port, which converts receivedoptical signals to electrical signals, amplifies them in the RF domain,and provides the electrical signals to a transmitter 129 that outputslight at, for example, 1610 nm, 1310 nm, or some other appropriatewavelength, provided to the WDM 122, which in turn directs the upstreamlight into fiber 118. At the head end 110, the fiber 118 is connected toWDM 116 that directs the upstream light to the receiver 114.

Each of the 32 ports of the splitter/combiner 126, through a respectivefiber 128, output a respective signal to a second activesplitter/combiner unit 130 of the same type and configuration as thesplitter/combiner unit 120. The length(s) of the fiber 128 may vary withrespect to each other. The output power per splitter port is low, around0 dBm. The splitter ports are connected to ONUs 140, for instance in aMultiple Dwelling Unit (MDU) or a neighborhood, via fiber 132 of lengthL3. In a basic RFoG system, the sum of the fiber lengths L1+L2+L3 is upto 25 km. The system 100, however, will permit a higher total length offiber between the head end 110 and the ONUs 140, such as 40 km, becausethe system 100 can tolerate a higher SNR loss, as further describedbelow.

The upstream signals from the ONU 140 are individually terminateddirectly at the active splitter/combiner unit 130; even for ONUsoperating at 0 dBm, the power reaching the detectors is around −2 dBm(the fiber 132 is a short fiber up to a few km, and the WDM loss insidethe active combiner is small). This is almost 20 dB higher than inexisting RFoG systems, meaning that the RF levels after the detector inthe splitter 134 is almost 40 dB higher than in existing RFoG systems.As a consequence, the receiver noise figure is not critical, and highbandwidth receivers can be used with relatively poor noise performance.The received RF signal is re-transmitted via the transmitter 136 alongthe reverse path into fiber 128 and received and retransmitted by thepreceding active splitter/combiner unit 120 and thereafter to the headend 110. Although the repeated re-transmission leads to some incrementalreduction in SNR, improvements in SNR from the active architectureprovides much greater overall performance relative to traditional RFoGsystems. More importantly, because all reverse signals are individuallyterminated at separate detectors, there can be no optical beatinterference (OBI) between different reverse signals. The reversesignals are not combined optically, hence OBI cannot occur.

Although in some embodiments, the RF splitter/combiner units such as 120and 130 may use an RF combiner to combine respective electrical signalsfrom each detector at each port, this may produce unacceptable losses inthe upstream transmission from the ONU to the head end. Therefore, theRF splitter/combiner units 120 and 130 preferably have the detectorsarranged in a transmission line structure such as shown in FIG. 14,which will not incur such high signal loss.

In the forward direction there may be multiple EDFAs, such as EDFA 124in the splitter/combiner unit 120; these EDFAs are cost effective singlestage devices with low power dissipation—typically 2 Watts or less.Cascading the EDFAs results in an accumulation of noise due to thefinite noise figures of the EDFAs. Whereas the active splitterarchitecture does not require the EDFAs, since an EDFA (not shown) in ahigh power head end 110 could still be used to provide power to the ONUs140, the use of EDFAs, such as the EDFA 124, inside the active splitterunits provides some advantages. For example, the complexity and powerdissipation of equipment in the head end 110 is greatly reduced, as isthe fiber count emanating from the head end 110. The amount of powerdelivered to the ONUs 140 is readily increased to approximately 0 dBmfrom −6 dBm in a traditional RFoG system. As a consequence, ONUreceivers obtain 12 dB more RF level from their detectors and do notneed as high a gain or as low a receiver noise contribution. Even withrelaxed noise requirements at the ONU receivers, the SNR impact due toEDFA noise is easily overcome due to the higher received power. Inaddition, more spectrum can be supported in the forward direction withan acceptable SNR relative to current architectures, such as 4 GHzinstead of 1 GHz in current RFoG, hence total data throughput rates cangrow significantly without a change in operation to permit for example,services that provide 40 Gbps download speeds and 10 Gbps upload speeds.

In some embodiments, the optical combiner provides upstream anddownstream RFoG capability and a completely transparent and reciprocalavenue for PON transmission. The optical combiner may enable completetransparency for PON deployments. For example, the optical combiner mayenable OBI-free and high capacity features by deployment in compatibleHFC D3.1 capable FTTH networks. Likewise, the optical combiner may beincorporated in to GPON, 1G-EPON, XGPON1, 10G/1G-EPON, 10G/10G-EPON. Thecompatibility with HFC and D3.1 enables the disclosed optical combinerto be deployed alongside a current HFC network, and is D3.1 ready. Theoptical combiner may be deployed on a fiber node, on multiple dwellingunit (MDU) and on single family home (SFU) deployments.

Embodiments for an RFoG combiner include preventing or eliminating OBIat the combiner as opposed to managing it at the extremities of thenetwork (such as using a CMTS scheduler at the head end side of thenetwork or wavelength specific ONUs at the subscriber end of thenetwork). Embodiments are described that enable elimination of OBI. Thedisclosed optical combiner may be used to eliminate OBI, enhancecapacity, and/or enable multiple services in RFoG, the cable version ofFTTH networks.

The disclosed optical combiner may be independent of ONUs, Cable Modemsand CMTSs. The disclosed optical combiner may be CMTS-agnostic, thuseliminating the need to create an RFoG-aware scheduler, which is bothrestrictive and time consuming. The optical combiner makes a cableversion of FTTH more feasible, as compared to the PON alternatives. Forexample, in embodiments, the disclosed optical combiner has a reciprocalPON pass-thru capability of the optical combiner along with a highupstream and downstream capacity, which assists RFoG deployment withoutinterruption to the underlying system, or impairing future inclusion ofPON functionality, such as later PON deployment on an RFOG system.

In some embodiments, the optical combiner has 32 ports, but onlyrequires one transmit port, one receive port, and one WDM component atthe headend. Thus, instead of requiring 32 WDMs and 32 receive ports,the disclosed optical combiner may save on head end space and power. Thecombiner may be an active device that needs approximately 2 Watts ofpower. The optical combiner may be powered by power sources readilyavailable in the RFoG system, or power can be provisioned into theoptical combiner. The power source may include a battery back-up orsolar/fiber power alternatives. If the power is lost and the battery hasalso drained, the entire reciprocal PON transmission is unaffected. Theupstream RFoG transmission is however stopped. In a conventional RFoGsystem it would have been stopped also because the preponderance of OBIwould have severely impaired the system anyway if the system was atraditional RFoG system with a passive combiner. Also in case of powerloss, ONU (Optical Networking Unit) at the homes would cease to functionsuch that without any power backup such systems will cease to function,whether those are RFoG or PON systems, with or without the activecombiner disclosed here. The head end optical receiver 114 may only needan input power range from 0 . . . −3 dBm, and require 15 dB less RFoutput power due to the absence of the RF combiner such that with such ahigh optical input power and low RF output power requirement the gaincan be low.

Disclosed are embodiments for an active optical combiner that receivesinput light from multiple downstream devices, that is, end user opticalnetworking units (ONUs) or downstream optical combiner devices andre-transmits RF signals modulated on the input light sources onto alaser that sends a modulated signal output upstream. In embodiments, theoptical combiner includes an optical splitter function.

As described above, a large service group (in this example of 32=32) canbe serviced by a single transmitter in the headend. In the reverse theONU sends out light with a power level Pr dBm into fiber L3 to a port ofthe Disclosed optical combiner unit. The 1=32 unit in the Disclosedoptical combiner unit is a passive splitter for forward signals but usesWDM components inside the 1=32 unit to split upstream light from the ONUoff from the fiber and directs that to photo-detectors such that allupstream paths are received. In embodiments, the photo-detector outputsmay be preferably summed in a transmission line structure and providedto an RF amplifier and laser with output power Pr3 that relays theupstream information to a WDM that combines the upstream light ontofiber L2 to the upstream optical combiner unit. The upstream opticalcombiner unit thus combines signals from multiple downstream opticalcombiner units and through laser transmitter with Pr2 output providesthe upstream signals to fiber Ll. The upstream signals on fiber Ll aredirected with WDM component at the headend to a headend receiver (HERx). Thus an overall system may use a single fiber from headend for avery large service group. This is enabled by an architecture withmultiple cascaded optical combiner units.

The disclosed optical combiner may preferably eliminate OBI, making anOBI-free system. The optical combiner enables long reach and largesplits, e.g. up to 40 km and 1024 splits, which will expand evenfurther. The high upstream and downstream capacity enabled by thedisclosed optical combiner includes up to 10G DS/1G US, and as high as40G DS/10G US.

In embodiments, the disclosed optical combiner prevents interference inRFOG deployments in the combiner rather than preventing interferenceusing measures taken in the ONU where previous attempts have failed orproven to be cost-prohibitive.

Traditional RFoG architectures have a fixed power budget. This meansthat as fiber length between the head end and the ONUs increases, asmaller number of splits may be used, as can be seen in FIG. 3 where thelower, curved line represents the existing architecture and the upper,curved line represents the active architecture disclosed herein.Conversely, the more splits that are desired, the less fiber length maybe deployed. The disclosed active architecture, however, enables fiberlength of up to approximately 40 km irrespective of the number of splitsused, meaning that the disclosed active architecture permits fiberlengths of 40 km or more along with a large number of splits, e.g. 1024,thereby advancing FTTP topology and deployment.

The overall cost of the active splitter architecture shown in FIG. 2 issimilar to that of a traditional RFoG solution. The cost of activesplitter EDFA gain blocks and WDM and detector components in the activearchitecture is offset by the elimination of head end gear such asreceivers, high power EDFAs and combiners. A cost reduction of the ONUsthat can operate with lower output power further supports the activesplitter architecture. Further advantages of the active splitterarchitecture may include a reduction in outgoing fiber count from thehead end, which can have a large impact on system cost, as well as anoption to use 1310 nm reverse ONUs while staying within a typical SNRloss budget, which can further reduce costs. Also, the system shown inFIG. 2 exhibits increased bandwidth relative to what existing RFOGarchitectures are capable of providing, avoiding limits on service groupsizes and concomitant requirements for more CMTS return ports. Finally,unlike OBI mitigation techniques in existing RFoG architectures, thesystem shown in FIG. 2 does not require cooled or temperature controlledoptics and bi-directional communication links that necessitateadditional ONU intelligence.

Each of these factors provides a further cost advantage of an activesplitter solution over existing RFoG architectures. Required space andpower in the head end is also reduced; the active splitter solutionrequires one transmit port, one receive port and one WDM component.Existing RFoG architectures, on the other hand, requires transmit ports,multi-port high power EDFAs, 32 WDM's, 32 receiver ports, and a 32-portRF combiner. Existing RFoG architectures require very low noise, highgain, and output power receivers with squelch methods implemented toovercome power loss and noise addition in the RF combiner. The system100 shown in FIG. 2, conversely, works with input power normally in the0-3 dBm range, little gain is required, and requires 15 dB less poweroutput due to the absence of the RF combiner before the CMTS.

Preferably, the disclosed optical combiner unit implements atransmission line approach to combine multiple optical photodetectors ina single optical receiver. This may be accomplished in unidirectional orbidirectional configurations. A unidirectional system provides nocontrol communication signals from an active optical splitter to an ONU,i.e. control communication signals only pass from an ONU to an activesplitter. Thus, in a unidirectional system, an active optical splittersimply accepts an output level from an ONU and operates with that outputlevel. A bidirectional system passes control signals from an activeoptical splitter to ONUs instructing them to adjust their output power;this type of system permits accurate equalization of the input levels tothe active optical splitter from each ONU.

Some active splitter/combiner systems may preferably include redundancywhere active optical splitters switch their return laser power (thereturn laser that carries the combined information of the ONUs connectedto it) between a high and a low power state or operates this laser in CWmode. In that case an upstream head end or active optical splitter caneasily detect loss of power at an input port and enable a second inputport connected to another fiber route to receive the information; in theforward path, the other fiber route would also be activated in this casebecause generally the forward and reverse light share the same fiber.Also, some active splitter/combiner systems may include a reverse laserin the active optical splitter that adjusts its power output as afunction of the number of ONUs transmitter to the active opticalsplitter and the photocurrent received from these ONUs. Still otheractive splitter/combiner systems may have a gain factor and reverselaser power of the active optical splitter set to a fixed value.

Preferably, the disclosed optical combiner unit is able to configureitself under changing circumstances. Instances occur in which cablemodems in the ONU are required to communicate with the CMTS even ifthere is no data to be transmitted. Usually, however, the ONU is turnedoff during periods when there is no data to be transmitted between theONU and CMTS, and a cable modem could go hours before receiving orsending data. Thus, in some embodiments the disclosed combiner unit maybe configured to stay in communication with the CMTS. Cable modems maybe required to communicate back to the CMTS once every 30 seconds, orsome other appropriate interval.

ONU Operational Modes and Laser Clipping Prevention

In traditional RFoG architectures, ONUs transmit information in burstsand at any point in time one or more ONUs can power on and begintransmitting information. As required by the DOCSIS specification, allONUs are polled repeatedly with an interval up to 5 minutes but usuallyless. When an ONU turns on, the optical power transmitted by the ONUrises from zero to the nominal output power in a short time. As aconsequence, the optical power received by the active splitter from thatONU goes through that same transition. The slew rate with which the ONUcan turn on is constrained by the DOCSIS specification, but thetransition is still relatively abrupt, resembling a step function. As iswell known from signal theory, a step function has a frequency spectrumthat contains significant energy in the low frequencies, with decliningenergy as frequency rises. If the low frequency energy were allowed tobe re-transmitted unimpeded by the active splitter laser whenretransmitting signals, then the signal could readily overdrive thelaser and cause laser clipping. To avoid such clipping, severalapproaches may be utilized.

First, a steep high pass filter may be implemented after the detectorsof the active splitter, which ensures that the low frequency signalsinduced in the photo detectors from ONUs that power on and off do notoverdrive the laser used for retransmission. Such a high pass filtershould be constructed so that it presents low impedance to the photodetectors for low frequencies, such that the photo detectors do not seea significant bias fluctuation when ONUs cycle on and off. For instance,if a coupling capacitor were used as the first element in a filter thatpresents high impedance to the photo-detectors, then an ONU that turnson could result in a significant bias fluctuation of the photodetectors; such a filter should preferably not be used. In this context,a significant bias fluctuation would be a fluctuation of greater than10%. Preferably, the high pass filter is configured to limitfluctuations to levels well below this figure, e.g. 5% or even 2%. Also,if the re-transmitting laser is used in burst mode, then the slew rateof the retransmitting laser should preferably be limited when it turnson, so as to limit the amount of low frequency spectrum into thephoto-detectors of preceding active splitter units.

As noted above, ONUs normally operate in burst mode and this causes theassociated problems just described. Burst mode operation of the ONUs isrequired in an existing RFoG architecture because otherwise, theprobability of OBI occurrence would be very high and the system wouldnot generally work. With the active splitter architecture, however, OBIcannot occur and the signal to noise margin is much higher than withRFoG. Because of this, a second approach to reducing clipping is tooperate ONUs in a continuous “on” state with the active architecturepreviously described. For 32 ONUs delivering signals into an activesplitter, the shot noise and laser noise accumulates, but the signal tonoise budget is so high that the resulting SNR performance is still muchbetter relative to existing RFoG systems. As a consequence, the activesplitter architecture allows operation of all connected ONUssimultaneously given that the active splitter architecture eliminatesOBI.

A third option to alleviate laser clipping is to allow the ONUs tooperate in burst mode, but to detect the amount of power out of the ONUand attenuate the ONU's signal so as to prevent clipping. Referring toFIG. 4, using a traditional RFoG system 200, the CMTS 210 may keep theRF level at a return input port constant. The return signal is generatedby a cable modem 220, provided to an ONU 230 that includes an opticalreverse transmitter and relayed over an optical network 240 to areceiver 250 co-located with the CMTS that converts the optical signalback to an RF signal and provides that to the CMTS 210. It should beunderstood that the optical network 240 can contain active and passiveelements. It should also be understood that the communication betweenthe cable modem 220 and the CMTS 210 is bidirectional, i.e. there areboth “forward” and “reverse” path signals.

The communication path shown in FIG. 4 may be used to adjust the outputlevel of the cable modem 220. In case the loss from the ONU 230 to thereceiver 250 is high, or the loss from receiver 250 to the CMTS 210 ishigh, then the CMTS 210 will adjust the output level of the cable modem220 to a high level in order to obtain a set input level at the CMTS ora level within a predefined range at the CMTS. In traditional RFoGsystems there is considerable margin on the input level that the ONU canhandle, to allow for this adjustment. However, it is still possible forthe cable modem 220 to overdrive the ONU 230, particularly as the amountof spectrum used by the cable modem increases to support future heavydata loads. When the ONU 230 is over-driven, then the RF signalmodulated onto the laser of the ONU 230 becomes so high that the reverselaser in the ONU 230 is driven into clipping, i.e. the output power fromthe laser swings so low that the laser is turned off. This causes severesignal distortions and creates a wide spectrum of frequencies thatinterferes with communication throughout that spectrum.

The optical network typically combines signals from multiple ONUs, eachONU is typically communicating in another band of the frequencyspectrum. The communication of all these ONUs is affected by the widespectrum induced by the distortions even if only one ONU is clipping.Preferably this problem is resolved in such a way that the other ONUsare not affected, the clipping ONU is brought to a state where it canstill communicate, and the CMTS produces a warning that an ONU is notoperating optimally.

A variation on the third option just described is to operate ONUs inburst mode where the ONU switches between a low power state (forinstance −6 dBm) and a high power state (for instance 0 dBm). This meansthat the ONU laser never fully turns off, i.e. the laser always operatesabove its laser threshold, and can always be monitored by the activesplitter. The reduction in output power when it is not transmitting RFsignals reduces the shot and laser noise accumulated in the activesplitter such that the signal to noise impact is minimized.

In circumstances where the optical combiner unit cycles to a low powerstate rather than a completely off state, the photodiode current and amax/min can be tracked for photodiode current across all of the ports ofthe combiner, and thus a microcontroller can be used at the opticalcombiner to continuously track the max and min in a specified timeinterval. For example, if for ten minutes the photodiode current max is0, then the optical combiner determines that the cable modem is eithernot connected, has a defective optical link, or is otherwise defective.Optionally the active optical combiner can signal absence ofphoto-current to a head end. The optical combiner is also able toconfigure itself whether or not the optical combiner can determine iflight received is bursty, as in normal RFoG operation, or CW (continuouswave) as with a node reverse transmitter. The optical combiner is ableto know by using CMTS upstream signaling imposed by the CMTS onto themodems to analyze which ports are working, which ports are silent, whichinput ports are connected to ONUs, and which input ports are connectedto optical combiner reverse transmitters, where optical combiner portsmay have an output power profile different from ONUs in the sense thatthe power may be CW or may be fluctuating between a low and a high powerstate or may carry information embedded in the signaling indicating thepresence of a further optical combiner between the ONU and the opticalcombiner.

For cascaded active splitters, the return lasers in cascaded activesplitters can similarly be operated in conventional burst mode where thelaser turns off between bursts, in CW mode, or in a burst mode thatswitches between a high and a low power state. It should also beunderstood that CW operation of reverse lasers and/or ONUs, or burstmode operation with a low and a high level further facilitatesdetermination of the optical input levels into the upstream input portsof active splitters. It should also be understood that, although thedevices and methods disclosed in the present application that prevent orotherwise reduce clipping by a laser operating in burst mode wasdescribed in the context of an ONU, the devices and methods used toprevent clipping by a laser in an ONU are equally applicable topreventing clipping by a laser in an active splitter as previouslydisclosed.

FIG. 5 shows a system that mitigates laser clipping that might otherwiseresult from burst mode communications from an ONU. Specifically, an ONU300 may include an RF rms detector 310, a microcontroller 320 and analgorithm to adjust an attenuator 330 in the ONU as a result of thepower detected at the RF rms detector 310. The reverse path from the ONU300 may be operated in burst mode; when an RF signal is presented to theinput 340 then the ONU's laser 350 is turned on by the bias circuit 360.This can be accomplished either by an additional RF detector (not shownin the figure) in the input circuit directly turning on the bias circuit(dashed arrow) or by the RF detector 310 and the microcontroller 320turning on the bias and setting the bias level. When a burst occurs, theRF detector 310 measures a power level and provides that to themicrocontroller 320. The microcontroller also is aware of the operatingcurrent of the laser 350 as set by the bias circuit 360. Thus, themicrocontroller 320 can compute if the RF signal level is large enoughto induce clipping of the reverse laser. If no clipping will occur, nofurther action needs to be taken and the ONU 300 can retain a nominal RFattenuation value. If, at that time, the ONU is not at a nominal RFattenuation value the procedure is more complicated, this will bediscussed later in the specification.

If clipping will occur, the microcontroller 320 stores the event. If aspecified number of clipping events has been counted within a specifiedtime interval, then the microcontroller 320 determines that the ONU 300is having significant performance degradation due to clipping, and isalso significantly impairing other ONUs in the system. In that case, themicrocontroller 320 computes how much the RF attenuation needs to beincreased to eliminate the clipping using RF power measurements thathave been previously recorded. The microcontroller 320 then increasesthe RF attenuation to a new value such that the laser 350 is modulatedmore strongly than normal (more modulation index than the nominalvalue), but still below clipping. The microcontroller 320 may optionallyalso increase the laser bias setting to provide more headroom for lasermodulation.

Because attenuation of the signal from the ONU 300 has been increased,the RF level as seen by the CMTS at the end of the link drops. The CMTSwill then attempt to instruct the cable modem to increase the outputlevel to restore the desired input level for the CMTS. This may resultin either of two scenarios. First, the cable modem may not be able tofurther increase output level and the CMTS will list the cable modem asa problem unit that is not able to attain the desired input level to theCMTS. This does not mean that the CMTS can no longer receive signalsfrom the cable modem, as the CMTS has a wide input range to acceptsignals. Hence, the reverse path still generally functions whereas itwould have been severely impaired had the clipping problem not beenresolved. Second, the cable modem may have more headroom, in which casethe CMTS will instruct it to increase its output level and restore theCMTS input level to the desired value. As a consequence, the reverselaser will be driven into clipping again and the ONU microcontrollerwill further increase the RF attenuation. This cycle will continue untilthe cable modem has reached its maximum output capability and then thesystem is back to the first scenario.

The system shown in FIG. 5 provides protection from clipping by ONUs,and also causes the CMTS to be aware of problem modems or ONUs. As waspreviously noted, the root cause of the problem was that the loss fromONU to CMTS was too large, due for example to a bad fiber connection inthe optical network from ONU to the receiver. This problem is signaled,and eventually will be fixed. When the problem is fixed however, theCMTS input level increases beyond the preferred CMTS input level andthen the CMTS will direct the cable modem to reduce output level. If theONU is not at the nominal attenuation value and notices that the actualmodulation index is at or below the nominal level then this can berecognized as different from the previous “new value” for ONUs that hadbeen over-driven that was deliberately set above the nominal modulationindex. This implies that the problem in the system has been fixed andthe microcontroller can reduce the attenuation down to the nominalvalue, gradually or in one step. Thus, this technique automaticallyrecovers from the state where it protects the ONU from clipping withincreased attenuation to nominal attenuation once the system has beenfixed.

As previously indicated, an ONU takes time to turn on after a burst hasbeen detected. For example, the RFoG specification indicates that theturn-on time of an ONU should be between 100 ns thru 1000 ns (i.e. 1μs). A turn-on time that is too fast undesirably creates a very high lowfrequency noise, which decreases as frequency increases. Unfortunately,because this noise extends to around 50 MHz or beyond, most of thecurrently deployable upstream signals are propagated within thefrequency range that is affected by noise due to an abrupt turn-on time.Exacerbating the signal degradation is the fact that the noise is spiky,in that the instantaneous noise burst could be much higher than what iscommonly seen on a spectrum analyzer with moderate video bandwidth.

FIG. 6 generally illustrates an ONU upstream architecture 400 where anRF detector 410 detects whether an RF signal is present at its input420. If a signal is detected, the RF detector 410 passes the signalthrough to an amplifier 450 and also signals a laser bias control module430 to turn on at time t0 a laser 440, which has a turn-on time 460. Theamplifier 450 amplifies the RF signal that is passed through from the RFdetector circuit 410. The amplified signal drives the laser 440. Thelaser's output is propagated from the ONU on a fiber 470. Forsimplicity, the downstream ONU receiver architecture is not shown inFIG. 6. The turn-on time 460 of the laser has a profound effect on thespectrum produced by the turn-on event.

FIGS. 7 and 8 show estimated spectra for a rise time of 100 ns and 1 μs,respectively, for a typical signal at 40 MHz. For a short rise time, thenoise due to the ONU turn-on is of the same order of magnitude as theintended signal. With a slower laser turn on this effect can bemitigated.

If there is just one ONU on at any given point in time, the effect oflow frequency noise due to ONU turn-on is negligible, because the DOCSISload is inset after the laser has fully turned on. However, when thereare multiple ONUs that can turn on at any given time, then the noise isoften not negligible. If there was a first ONU on and a second ONU turnson while the first one is transmitting data, then the spikes in highnoise, described above, are present across a wide range of the frequencyspectrum of the upstream signal while the first ONU is transmittingdata. Depending upon the relative RF levels of the signals and themagnitude of the noise spikes, the signal may experience pre-or evenpost-forward error correction (FEC) errors, when measured at the CMTSfor example. The potential for debilitating noise becomes more and morepronounced as the numbers of ONUs that can turn on increases, as islikely to happen as architectures migrate to the DOCSIS 3.1 standard.While this problem has always existed, it only becomes apparent, as aresidual error floor, when the OBI and its induced errors areeliminated.

An additional impairment is caused by the application of the RF signalbefore the laser has fully turned on and has stabilized. Specifically,an impairment can occur for example if the laser turn-on time is slowerthan the DOCSIS Preamble which may be applied before the laser hasreached steady state. Typically, the DOCSIS Preamble is sent as a QPSKsignal and can often be 6 to 10 dB higher than the regular RF signalthat follows, depending upon signal conditions. In such an instance, thelaser will be over-driven while still in a low power state andexperience very large clipping events that may cause spikes in noisethroughout the RF spectrum of the upstream signal, and thus hide othersignals that may exist at the same time. As previously indicated, whilethis effect has always occurred, it only becomes observable with theelimination of the OBI, and its attendant OBI-induced errors.

FIG. 9 shows a bias, around which a laser is modulated with a sine wavesignal. During the time that the laser bias is insufficient, the outputsignal is clipped. For slower laser turn-on, the duration of theclipping is increased. While it may be desirable to reduce the lowfrequency RF spikes that occur across the upstream frequency spectrum byhaving a slower turn-on time, the increase in clipping described abovemay counteract the benefit of the slow turn-on time. Disclosed are noveltechniques that permit a slow turn-on time while avoiding clippingartifacts.

Referring to FIG. 10, a novel ONU upstream architecture 500 includes anRF detector 510 that detects whether an RF signal is present at itsinput 520. If a signal is detected, the RF detector 510 passes thesignal through to an amplifier 550 and also signals a laser bias controlmodule 530 to turn on at time t0 a laser 540, which has a turn-on time560. The laser bias control module 530 preferably modulates the bias ofthe laser 540 to achieve a full turn-on of the laser 540 over a turn-ontime 560 that is preferably as slow as possible, e.g. the slowestturn-on time allowed by the RFoG standard, or in some embodiments evenlonger. In some embodiments, the turn-on time of the laser 540 could beup to 500 ns, 1 μs, or longer. This may greatly reduce the low frequencynoise. The turn-on time for the laser may be linear, as shown in FIG.10, or may implement a transition along any other desired curve, such asa polynomial curve, an exponential curve, a logarithmic curve, or anyother desired response.

The amplifier 550 amplifies the RF signal that is passed through fromthe RF detector circuit 510. The amplified signal drives the laser 540.Preferably, when amplifying the RF signal from the RF detector 510, thelaser bias control module 530 includes a circuit that modulates theamplifier gain to be proportional to the laser bias. This effectivelysets the gain of the amplifier 550 to be proportional to the laserturn-on 560, and thereby reducing or even preventing over shoot andclipping by the laser 540. The laser's output is then propagated fromthe ONU on a fiber 570.

FIG. 11 shows the output of the laser 540 when using the system of FIG.10. As seen in this figure, when using an RF gain factor proportional tothe laser bias, the clipping no longer occurs. However, the variation inRF level during the laser turn-on may potentially cause an issue in theburst receiver that may expect a near constant RF level during the laserturn-on. To mitigate this, in some embodiments, the amplifier bias maybe modulated to delay the RF signal to the laser, relative to theturn-on time of the laser 540, and may also apply a faster time constantthan the optical power turn on. This embodiment is illustrated in FIG.12.

FIG. 13 shows an implementation of an ONU that includes a delay in theRF signal to the laser, relative to the turn-on time of the laser, andalso applies a faster time constant than the optical power turn-on.Specifically, a novel ONU upstream architecture 600 includes an RFdetector 610 that detects whether an RF signal is present at its input620. If a signal is detected, the RF detector 610 passes the signalthrough to an amplifier 650 and also signals a laser/amplifier biascontrol module 630 to turn on at time t0 a laser 640, which has aturn-on time 660. The laser/amplifier bias control module 630 preferablymodulates the bias of the laser 640 to achieve a full turn-on of thelaser 640 over a turn-on time 660 that is preferably as slow aspossible, e.g. the slowest turn-on time allowed by the RFoG standard, orin some embodiments even longer. In some embodiments, the turn-on timeof the laser 640 could be up to 500 ns, 1 μs, or longer. This maygreatly reduce the low frequency noise. The turn-on time for the lasermay be linear, as shown in FIG. 13, or may implement a transition alongany other desired curve, such as a polynomial curve, an exponentialcurve, a logarithmic curve, or any other desired response.

The amplifier 650 amplifies the RF signal that is passed through fromthe RF detector circuit 610. The amplified signal drives the laser 640.Preferably, when amplifying the RF signal from the RF detector 610, thelaser/amplifier bias control module 630 includes a circuit thatmodulates the amplifier gain to be proportional to the laser bias, butwith a delay 680 relative to the time t₀ that the laser 640 begins toturn on. Preferably, the rise time of the amplifier gain is faster thanthe rise time of the laser turn-on. In some embodiments, thelaser/amplifier bias control module 630 simply switches on the RF gain,i.e. the rise time is as short as the amplifier allows. The laser'soutput is then propagated from the ONU on a fiber 670.

This ONU shown in FIG. 13 effectively sets the gain of the amplifier 650to be proportional to the laser turn-on 660, and thereby reducing oreven preventing over shoot and clipping by the laser 640, while at thesame time mitigating problems caused by a receiver expecting anear-constant RF level during the time that the laser turns on. Theability to simultaneously reduce the laser turn-on time and to providean RF gain to the laser in proportion to the laser turn-on time, butdelayed with respect to the laser turn-on time is a feature that hasgreat potential in all applications, and without loss of generalitythese techniques may be used for any analog application such as DOCISIS3.0 or 3.1.

Either (or both) of the architectures shown in FIGS. 10 and 13 may beused together with the architecture shown in FIG. 2 so as to furtherimprove speed and stability of HFC systems. These may further be usedtogether with the long term clipping reduction discussed in the previousdisclosure to reduce the effects of both long term and short termclipping in the system.

Burst Detection

As indicated earlier, upstream transmissions typically operate inburst-mode (BM), where ONUs power up a transmitter, e.g. a laser, onlyduring time intervals when information is to be transmitted along theupstream path. A burst-mode system generally provides a lower noiseenvironment and thus enables better SNR, and in the case the transmitteris an optical device, the use of burst-mode tends to reduce Optical BeatInterference (OBI). Thus, in some preferred embodiments of the opticalcombiner system previously disclosed in this specification, where OBI isto be suppressed, such optical combiners are preferably operated inburst mode.

Also as indicated earlier, RFoG architectures that use burst-mode detectthe RF level in the ONU, powering the ONU's laser when an RF signal isdetected and powering down the laser when the RF signal is not present.This procedure is referred to as “RF detection.” In an optical combiner,the optical light inputs coming from the ONUs are all detected and thedetector outputs are collected. If RF detection is used with an opticalcombiner, an RF comparator would be applied to the output of thecombined RF output. If the RF level output of the combined RF detectorswere higher than the applied comparator, then the optical laser in theoptical combiner would be activated.

However, such detection may be fraught with difficulties because the RFlevel input could be very small. For instance, a very small slice of aD3.1 signal could be produced by any single ONU, hence the modulationindex of the ONU would be low, resulting in a low RF level at theoptical combiner. Also, optical input power to the optical combiner froma given ONU could be low; with an optical input range spanning up to 12dB, the RF level after detection could vary by 24 dB. As a result, theRF level from a photodiode could still be so low that the RF level thatis to be detected would be lower than the comparator, even if the RFlevel were high relative to the Optical Modulation Index of the ONUlaser that generated the RF signal. In ONU embodiments, the RF levelcould be turned on after the optical output is turned on, or while theoptical output is being turned on, such that the detection of an RFlevel at the disclosed optical combiner would be delayed. Furthermorethe detection could also be slow, because it depends upon the comparatorcircuit.

An alternative to using burst detection on the cascaded optical combinerunits disclosed in the present application would be to keep the upstreamlight transmission on all the time, irrespective of whether signals areprovided to the optical combiner or not, i.e. an “always on opticalcombiner”. Though this would ensure that the optical combinertransparently relays information upstream, it would result in a constantlight input at all the ports at an upstream optical combiner device ormultiple port receiver. The total light input at the ports thus couldlead to a summation of shot noise from all the ports, degrading the SNRperformance of the total system. For this reason, in preferredembodiments, the optical combiner unit transmits upstream light onlywhen an RF signal has been received and is to be sent out.

Disclosed herein is a novel method of burst detection that is fast,simple, stable and robust thus enabling multiple new architectures.Specifically, broadly stated, the disclosed optical combiner system maymonitor the optical current of each photo diode as well as the sumcurrent of all photodiodes. If any one of the photo diodes registers aphoto current, or alternatively a current above a certain minimum value,the retransmitting laser is automatically turned on. The photodiodecurrent generation is instantaneous and beneficially is a DC value thatis easier to compare. As speeds of the interconnecting networks increaseover time, such optical detection circuits will become more useful.

Such an Optical Burst Mode (OBM) detector promotes reliability and mayhave the following advantages: (1) in the case of multiple daisy chainedoptical combiners as disclosed in the present application, substantialreduction in the additive shot noise is achieved relative to an “alwayson” solution; (2) in the case of DOCSIS 3.1 transmission, individualsignal transmissions with very low RF levels per ONU may be detected andretransmitted; and (3) in the case of varying optical input levels dueto different optical lengths between the ONUs and the disclosed activeoptical combiner, or varying optical lengths between multiple daisychained such active optical combiners, reliable burst mode operation maystill be achieved.

Furthermore, the disclosed novel burst detection also enables detectionof light at the input immediately at the start of a burst at the opticalcombiner input. Conversely, where there is no light at the input, oralternatively no light for a certain period of time, the ancillary RFamplifiers in the disclosed active optical combiner may be powered down,thus reducing the power dissipation of the disclosed active opticalcombiner. When light appears at the input of the disclosed activeoptical combiner, the amplifiers can be powered on again within the timeallowed; for instance in an RFoG system up to one microsecond is allowedto establish an optical link from the moment that the RF input isdetected and the system has started to turn on. Because RF amplifierstake a finite time to turn on and establish amplification; earlydetection of a burst is important to provide enough time to establishnormal operation. Such power cycling could reduce power dissipation byas much as ten times, thus drastically improving the criticalinfrastructure metrics. Thus, for example in the event of a poweroutage, the optical combiner can conserve the power required by not onlyusing optical burst operation, but also circuitry for RF burstoperation, and extend a battery's life, if available.

Implementation of an optical power detection circuit capable of coveringa wide range of optical input power, in an architecture having multipledetectors is not trivial. Given the large number of detectors present,combined with a wide optical input power range, the amount and range ofphotocurrent that needs to be reliably detected is considerable. Simplymeasuring the voltage drop across a resistor in the detector biasnetwork is difficult; at low input power on a single detector, a smallvoltage drop can be reliably detected only if the value of a resistor,across which is a voltage drop equal to the photodetector bias, isrelatively high. However, increasing the value of such a resistor is notdesirable because this leads to an increased voltage drop when highdetector currents are present at multiple detectors; the detector biaswould become a strong function of the optical light present at thedetectors. In some embodiments, the detector bias is held constantbecause detector responsivity depends on detector bias; thus a varyingthe detector bias could lead to a variation in the gain of the system.Even a resistance value as low as a typical transmission line impedance,such as 75 Ohms, can be problematic when a large number of detectors areactive, and for instance 100 mA of detector current flows in themultiple detector system, leading to an excessive drop in detector bias.

Disclosed is a method to detect optical light over a wide input powerrange while retaining a constant bias on the detectors present in thetransmission line receiver. In order to accomplish this, a combinationof both an RF amplifier and a trans-impedance amplifier are used withthe multiple detector structure. In some embodiments, thetrans-impedance amplifier is connected to a high-pass structure in frontof the RF amplifier such that for low frequencies the trans-impedanceamplifier has a very low impedance connection (less than thetransmission line impedance) to the detector bias.

Referring to FIG. 14, which shows an example of a transmission linereceiver structure 700, a photo-detector may be accurately modeled up tofairly high frequencies (˜1 GHz) by a capacitance in parallel with acurrent source for reasonable input power levels (>1 uW). Thus, in thisfigure, each of the circuit elements 710 would be a model of aphotodetector. Conventional receiver designs use a trans-impedanceamplifier or match the detector to as high an impedance as possible,such as 300 Ohm, so as to convert the current source signal to an RFsignal with the best possible noise performance. These approaches arelimited by the detector capacitance such that an increase in the numberof detectors by simply combining detectors or detector area leads to aloss of detector performance due to an increase in combined detectorcapacitance, and therefore a large number of detectors (e.g. 32) cannotreasonably be expected to work well with a single RF amplifier. Thisimplies that multiple amplifiers are needed to receive a large number offibers.

As an alternative, multiple detectors could be provided to an RFcombiner before being amplified. An RF combiner requires that eachdetector be terminated individually with an RF impedance that istypically less than 100 Ohm, which will consume half of the detectorcurrent and, due to combining signals from multiple detectors, the RFcombiner will introduce an additional loss of at least 10*log(N) dB,where N is the number of detectors combined. This loss becomes excessivefor 8 detectors or more. Further, other losses are caused by practicalimplementations of RF combiners that require expensive transformers intheir realization. The transformers also cause bandwidth limitations andaforementioned other losses, and are difficult to implement for highimpedances (such as greater than 100 Ohm).

In the disclosed transmission line receiver, use is made of the insightthat a reverse biased photo-detector behaves as a current source inparallel with a capacitor with a low loss at RF frequencies. Thistransmission line receiver will not induce the 10*log(N) loss of the RFcombiner, not require transformers, offer a high bandwidth and be ableto provide an output signal representative of a delayed sum of a largenumber of detectors. A transmission line with impedance Z can be modeledby a ladder network of inductors and capacitors with L/C=Z², which workswell for frequencies under the resonance frequency of L and C. Practicaldetector capacitance values are on the order of 0.6 pF, such that a 75Ohm transmission line would require L=3.4 nH. The resonance frequency iswell over 1 GHz such that, for up to 1 GHz, a transmission line with anarbitrary number of detectors compensated with 3.4 nH inductors wouldsimulate a 75 Ohm transmission line. The quality of the parasiticcapacitance of the reverse biased detectors is such that they can beconsidered low loss capacitors at RF frequencies. The 3.4 nH can also bedistributed around the detectors as 2×1.7 nH, leading to a design asshown in FIG. 14.

As indicated above, each current source/capacitor combination 710represents a detector. FIG. 14 shows a number of these in series,separated by respective transmission line sections 720 (100 psec or onthe order of 1 cm on board) having 75 Ohm impedance. The detectors arematched with 1.7 nH inductors 730. A 75 Ohm resistor 740 terminates theinput of the transmission line. The output 750 of the transmission linefeeds a low noise 75 Ohm RF amplifier (not shown). It should beunderstood that, although FIG. 14 shows six detectors, there is no limiton the number of detectors that can be combined by concatenating thesesections, and up to the LC resonance frequency there is negligibleimpact on the attainable bandwidth for a large number of detectors. Inpractice the 1.7 nH inductors could be implemented in the PCB layout asnarrower line sections, and a balanced transmission line with 100 Ohm or150 Ohm differential impedance may be used to slightly improve noisefigure.

As shown in FIG. 14, each current source/capacitor combination 710represents a photo detector, where the current source is the detectedcurrent in the detector; and the capacitor represents the parasiticcapacitance of the detector. Multiple detectors are connected withsections of transmission line (such as T2) and matching inductors (suchas L1 and L2). The matching inductors are chosen such that the parasiticcapacitance of the photo detectors is matched to the transmission lineimpedance (typically 75 Ohm). Thus, multiple detectors can be connectedand concatenated to a transmission line, such that the detector currentsare provided to the transmission line and these detector currents areequally divided to propagate both to the output 750 and to thetermination resistor 740 at the other end of the transmission linestructure. Each detector current generally passes through transmissionline sections, matching inductors, and detector terminals beforereaching an end of the transmission line. Thus, signals from adjacentdetectors affect the signal voltages present at each detector terminaland could therefore affect the detector current itself, causing across-modulation of detector signals. However, because a detector atreverse bias can be modeled as a good current source, such across-modulation does not occur. Each detector current half is thuspresented at the output of the transmission line as a signal with adelay proportional to the distance of the detector to the output of thetransmission line. This distance determines the delay of an electricalsignal at the terminal of the detector to the output of the transmissionline and includes delay due to matching inductors and photo-detectorcapacitance. The signal at the output of the transmission line istherefore proportional to the sum of the delayed detector currenthalves, independent of the number of detectors in the transmission linestructure. The signal at the output of the transmission line can thus besaid to represent the sum of the delayed detector currents.

The transmission line structure bandwidth is limited only by theinductive matching of the photo-diode capacitance and can be very large,exceeding 1 GHz. The output 750 is connected to an RF amplifier matchedto the transmission line impedance, which amplifies the signals outputfrom the transmission line structure. Note that use of a trans-impedanceamplifier that is not matched to the transmission line structure wouldcause a very large reflection of the output signals back into thetransmission line structure; a trans-impedance amplifier is not apreferable means to amplify the output from a transmission linereceiver.

Typically the photo detectors need to be biased, for instance with 5 V.In order to decouple the bias voltage from the amplifier, a decouplingcapacitor may typically be used. The bias can then be provided via aninductor in a bias-tee arrangement as shown in FIG. 15, for example. Thesignal from the transmission line 760 is provided to an amplifier (notshown) via a capacitor (770) that passes high frequency signals, andbias from a voltage source 775 is provided to the transmission line viaan inductor 780 that passes low frequency signals. The terminationresistor 740 at the other end of the transmission line is thuscapacitively decoupled to permit a DC bias. The current through voltagesource 775 can be measured to determine photocurrent; the voltage source775 could be constructed as a trans-impedance amplifier providing aconstant voltage and an output proportional to the current provided.However, in implementations, the inductor 780 needs to be chosen with avalue large enough that it does not affect the low frequency response ofthe amplifier. As a consequence, there may be a delay in the response ofthe current in the inductor 780 to a change in photo detector current,and this tends to cause a delay in the detection of photocurrent.

FIG. 16 shows an implementation 800 that uses both ends of thetransmission line receiver structure to alleviate such a delay. Theresistor R1 in FIG. 16 is the termination resistor 740 shown in FIG. 14,and the inductor L1 is the inductor 780 in FIG. 15. The voltage source810 provides bias both to the termination resistor 740 and the inductor780. The current in resistor 740 responds instantly to a photocurrentsuch that a fast detection of photocurrent is enabled. The inductor 780can support large photocurrents without a significant voltage drop suchthat large photo currents can be supported without a significant drop inbias to the photo detectors. A capacitance 815 can be placed adjacent tothe voltage source 810; for an ideal voltage source it may not carry anycurrent because the voltage is constant. However at RF frequencies itcan be difficult to realize a perfect voltage source, hence thecapacitor 815 provides a low impedance to ground such that RF currentsin the termination resistor 740 do not cause modulation of the voltageat the voltage source 810.

In order to realize an efficient detection circuit for the current involtage source 810, the voltage source 810 is preferably implemented asa trans-impedance amplifier. A trans-impedance amplifier is a basicelectronic circuit that holds a node between two current paths at aconstant voltage and has an output that changes its output voltage inproportion to the current provided at that node. Thus, externally thetrans-impedance amplifier looks like a voltage source to that node, butthere is an additional output that represents the current provided. Thisoutput may then be used to drive a decision circuit to decide if aphoto-current flows or not. Due to the fact that the trans-impedanceamplifier is realized with a practical transistor circuit, it does nothave infinite bandwidth, which means that it is not able to hold thenode voltage constant for very high frequencies and for that reason thecapacitor 815 may be added in some embodiments.

It should be understood that in some embodiments, the LC bias networkprior to the amplifier (capacitor 770 and inductor 780) may be replacedby more complex circuits, or even with diplex filters—provided that thenetwork provides a low-loss, high-frequency path from the transmissionline detector to the amplifier, and a low-loss (low impedance) path atlow frequency from the voltage source (trans-impedance amplifier) to thetransmission line detector bias. It should also be noted that thetrans-impedance amplifier may be implemented such that the outputvoltage first changes linearly as a function of photo-current, but thensaturates at a photo-current that is sufficiently high.

In other implementations, a photocurrent detection circuit may beapplied to each individual photo detector; optionally one electrode of aphoto detector (for instance cathode) may be connected to an RF circuitand the other electrode (for instance anode) may be connected to anoptical power detection circuit. This increases complexity, as adetection circuit is required per detector. Also, some embodiments mayoptionally use a trans-impedance amplifier per detector.

With an optical burst mode detection circuit, for instance of the typedescribed above, the bias of a laser or the bias or gain of an amplifiermay be controlled. FIG. 17 shows a multiple-detector receiver 820 thatproduces an output 825 signaling that power has been detected from anyone of multiple inputs 830. This detection can be based on a detectionmethod as described in the previous section or on multiple detectorcircuits that are monitoring individual detectors 835. When opticalinput has been detected at time t0 then the laser bias is turned on witha controlled rise time t_on_1 and the active combiner can re-transmitsignals present at the inputs.

The optical burst mode detection can further be used to control theamplifier bias as shown in FIG. 18; when optical power is detected at t0the amplifiers are immediately turned on. The laser turns on more slowlysuch that the amplifiers are settled by the time that the optical poweris on. Optionally this scheme may be expanded by a third control signal850 that controls the amplifier gain, as shown in FIG. 19.

Optical Modulation Index and Self Calibration

For implementations that permit operation of all upstream inputs of theactive splitter simultaneously, the total amount of photocurrent on thedetectors following the upstream inputs can be high. The impedance ofthe bias circuit and, as discussed, of the aforementioned filteringmeans in the detector output path must be low.

In an existing RFoG system, the CMTS controls the output level of thecable modems' communications with ONUs that are transmitting RF signalsto a head end such that a desired input level to the CMTS is obtained.This implies that the output level from a receiver preceding the CMTS isadjusted to a known level. If this receiver is of a type that has aknown amount of gain such that an output level corresponds to a knownoptical modulation index, then this implies that the optical modulationindex of channels provided to the CMTS is known—given the RF signallevel to which the CMTS adjusts the channel. This requires a calibratedreceiver that adjusts its gain as a function of the optical input level(2 dB gain increase for every dB reduction in optical input level) suchthat this fixed relation between RF output level and optical input levelis maintained. The modulation index into the receiver is the modulationindex of the upstream laser in the active splitter connected to thatreceiver; thus the CMTS implicitly controls the modulation index of thatactive splitter output.

The gain of the active splitter should preferably be set such that anoutput modulation index from that active splitter has a known relationto an input modulation index at one or more of the photo detectorsreceiving upstream signals from active splitters or ONUs furtherdownstream. This requires knowledge of the photocurrents at these photodetectors, and preferably the active splitter can monitor the photocurrent of each upstream link by using one detector per upstream link asin a transmission line detector, for instance. Since some systems mayoperate in burst mode, these photo currents are not always available.However, in a DOCSIS system all ONUs are polled repeatedly to obtain anacknowledgement signal with an interval up to five minutes. This impliesthat upstream active splitters are re-transmitting the information, andall active splitters in such a system have each one of the upstreaminputs active at least once every five minutes. The active splitter canthus record the burst levels and build a map of optical input levels toinput ports. Using this information, the active splitter can set aninternal gain level such that the upstream modulation index ismaximized, but will not clip so long as the input signals to the activesplitter are not clipping. Whereas the fiber length from head end tofirst active splitter is generally long, those fiber lengths betweenactive splitters and those fiber lengths from active splitters to ONUsare generally short, and have small enough loss that the optical inputpower values to the different upstream input ports are close, and theoptimal gain setting is similar for all ports. As a consequence, theoptimal gain setting in the active splitter is almost the same for allinput ports and the compromise in SNR from assuming a worst case reverselaser modulation index from a signal on any of the input ports is small.

As noted earlier, one embodiment could use the high and low opticaloutput power setting for the reverse laser, instead of switching thelaser between a high output power for burst transmission and an offstate in between. Not only does this embodiment provide continuousinformation to active splitters about the link loss to the ONU, it alsoimproves laser operation. When a laser powers on, the transient leads toa brief transition where laser distortion is high and RF input signalscan be clipped. If a laser is held at a low power level instead of beingin the off state before being turned on to a higher power level, thenthis transient is near absent and distortions and clipping are reduced.In case the laser is held at a high output power continuously, thesetransients and distortions are absent. The active splitter architecturepermits operating the ONUs in any of these three modes and an optimumcan be selected for system operation.

Whereas the upstream input power levels to detectors on an activesplitter are typically similar, in some instances they may differ due todifferences in connector loss or fiber loss. Preferably, all opticalinputs would have the same level or have the same RF level following thedetector for an equivalent channel load. Since the active splitter canmonitor the power level at each detector and map those optical inputlevels, it can compute adjustments to optical input power level or inmodulation index of those inputs that would be required to equalize theRF levels following the detectors of each input. The active splitter cancommunicate those preferred settings for output power level or gain forthe reverse transmitters downstream that are connected to the inputs.The communication signals can be modulated onto a laser injected intothe downstream signals or onto pump laser currents in EDFAs amplifyingdownstream signals. The modulation can be selected to be small enough,and in such a frequency band, that the communication signals do notinterfere with the downstream payload.

Preferably, not only active splitters receive and interpret thesecommunication signals, but also downstream ONU units receive andinterpret the signals. This would permit essentially perfect alignmentof the optical transmission level and RF gain of all units in an activesplitter system. Given the presence of an upstream laser, and theability of all components in an active splitter system to receive anupstream signal, all components in an active splitter system are capableof upstream communication with the addition of a simple tone modulationor other scheme. Thus, bidirectional communication is enabled, andactive splitters and the head end can communicate with each other,self-discover the system, and setup optimal gain and optical levels.

One objective of the active splitter architecture is to provide accurateRF levels to the CMTS that represent an optical modulation index. Doingso is not trivial, and requires a specific self-calibration procedure(later described) that is expected to result in accurate modulationindex correlation to active splitter head end receiver output RF levels.The receiver is either a CMTS plug-in or is connected directly to theCMTS without unknown RF loss contributions in between (in case a tap isneeded for other services than the CMTS, the tap can be integrated inthe receiver to avoid external RF losses). As a consequence, themodulation index of the active splitter re-transmitter units is setprecisely.

In case bidirectional communication is not available then the ONU outputpower level cannot be adjusted by the active splitter and the modulationindex of the ONU will still have some uncertainty since the optical lossbetween ONU and the active splitter/receiver can vary; a +/−1 dB lossvariation from ONU to active splitter would result in a +/−2 dBtolerance in RF level, thus a dynamic window will at least have toaccommodate that variance and headroom for other tolerances and CMTSsetup accuracy. This should be readily available for bandwidths up to200 MHz such that even without the active splitter controlling the ONU'soutput power, acceptable system performance can be obtained. With theaforementioned bidirectional control additional system headroom can beachieved.

When 1200 MHz return bandwidth is used, such that ONUs are assigned 200MHz widths of spectrum, the ONUs can all be operated a few dB belowtheir clip point, i.e. just enough to cover the uncertainty in the lossfrom the ONU to the active splitter to avoid clipping of the ONUs. Thisoptimizes the performance of the critical link from the ONU to theactive splitter, so that 0 dBm ONUs are sufficient. In this type ofoperation, an arbitrary choice can be made for the number of ONUsoperating with such a 200 MHz band, for instance up to six ONUs. This inturn would cause clipping in the active splitter transmitter; thus for1200 MHz operation, the gain of the active splitter receivers followingthe ONUs can be reduced by 8 dB such that when six ONUs are transmitting200 MHz of signal bandwidth, the active splitter reverse transmitter isoperated just below clipping. This method of operation maximizes SNR andeliminates uncertainty—the impact of variance in the link between theONU to the active splitter is minimized, and the active splitter linksare operated with a precise modulation index as with lower bandwidth RFreturn systems. The required dynamic window is reduced to tolerances inCMTS level setting and active splitter output level calibration,permitting operation at an optimal modulation index.

Analysis of the attainable SNR using the system just described, for 1200MHz operation with a maximum load of 200 MHz per ONU, results in a 5 dBimprovement in the SNR attainable at 1200 MHz. This results in about 20%more throughput capacity in the system. With 1200 MHz of bandwidth, thetotal upstream data rate could be as high as 10 Gbs.

In case the system is initially set up so that the active splitter unitsexpect a 1200 MHz return spectrum (instead of for instance 200 MHz) witha maximum of 200 MHz per ONU, then a penalty of around 7 dB occurs interms of peak NPR performance. Therefore, the mode of operationpreferably can be switched between normal operation, where a single ONUcan occupy the entire spectrum, and high bandwidth operation where asingle ONU can be assigned a limited amount of spectrum at any time andthe active splitter reverse transmitters support the entire spectrum atonce.

The proposed architecture has multiple re-transmission links that arepreferably operated at the best possible modulation index on theassumption of perfect alignment of the NPR (Noise Power Ratio) curves ofthose links. As noted earlier, the alignment of the re-transmission inthe active splitter return links is critical to obtain the best possibleperformance (every dB of mis-alignment directly results in a reductionof available SNR) hence a calibration technique is needed to set andhold the correct alignment of transmitter gain factors.

In order to provide such calibration, the active splitter returntransmitter gain will be set accurately, such that for a given detectorcurrent of the active splitter receiver diodes, the modulation index ofthe transmitter is equal to the modulation index input to the detector.This only requires knowledge of the detector current; the actual opticalinput power to the detector and the detector responsivity areirrelevant. In order to accomplish this, means are implemented at eachdetector to measure detector current such that an appropriate gain canbe set for the return transmitter.

The gain may be set individually for each detector, but since multipledetectors can be receiving signals at the same time, this would requirea controllable attenuator for every detector (32 detectors are in atypical active splitter unit). Preferably, a single attenuator is usedfor all detectors. This is achieved using variable output transmittersin the active splitter units, communicating to an upstream activesplitter or variable output transmitters in ONUs communicating to anupstream active splitter. Outlined below is a method to set the outputlevel of each of the reverse transmitters such that each transmitterprovides the same photocurrent on the detector to which it is coupled.During normal operation, the active splitter receiver monitors thedetector currents during bursts to enable issuance of a warning in casean optical link degrades or is lost.

For a 1310 nm reverse link from the active splitter to an upstreamactive splitter, the reverse laser power typically needs to becontrolled from either 3-10 dBm or 6-10 dBm, depending on the design ofthe active splitter receiver. For a 1610 nm reverse link, these figuresare typically 3-7 dBm or 6-7 dBm, respectively. These controls ensurethat the power received at the end of a 25 km link, with some WDM loss,is at least 0 dBm. It should be understood that the numbers given areexamples. The active splitter can transmit information in the forwarddirection through pump modulation of the EDFA or injection of a signalinto the forward path. The latter is more expensive; the former resultsin a lower data rate, as only a minimal pump fluctuation can be allowedwithout affecting the forward path. A low data rate is sufficient, andcan be read by a simple receiver—for instance a remote controllerreceiver operating in the kHz range coupled to a low cost processor. Itshould be understood that the downstream transmit function is onlyrequired in upstream active splitter units unless ONUs are beingcontrolled as well. In the figures shown, that would be one out of 33active splitter units in the system.

In a self-calibration run, the upstream active splitter unit transmits acommand downstream to active splitter units to initiateself-calibration. Subsequently the downstream units randomly turn theirtransmitters on and off at full power with a low duty cycle, such thatin nearly all cases at most one of the downstream units is on. Theupstream active splitter reports information downstream as to which portis on, and what detector current it has obtained from that unit. Thedownstream units record that information in non-volatile memory; sinceit can correlate the messages to its own activity, this providesinformation to the downstream unit as to what port it is on and whatpower it provided to that port. After all ports have been on at leastonce, or a time out has occurred (for instance if one or more ports arenot connected), the upstream active splitter unit determines whichdownstream active splitter produces the smallest detector current. Next,the upstream active splitter computes how the upstream powers of each ofthe downstream units should be set, such that all detector currents arethe same and fall within a specified range. That range can for instancecorrespond to 0-3 dBm (or 6 dBm) input power at the detectors. It shouldbe understood that this can be accomplished by setting a photodetectorcurrent, and does not require measurement of an exact optical inputpower.

Generally, the active splitter upstream unit will set this power to thebest (or maximum) value that can be obtained to optimize the SNR of thelinks. The active splitter units will then all have a known outputpower, and their internal gain will accordingly be set to have acalibrated modulation index for a given input power and modulationindex. All links into an upstream active splitter may behaveidentically. The upstream active splitter unit may then take thedownstream units out of calibration mode.

In case an additional port is lit up on an upstream active splitterreceiver port, then the self-calibration algorithm can proceed withoutservice interruption of already connected active splitter units. This isachieved by activating self-calibration on the downstream activesplitter receiver that has just been activated by requesting calibrationmode only for units with unknown port number (that is only the newunit). Its output will turn on and the upstream active splitter unitwill then assign a port number to the new, hitherto unused port and seta power to the new unit, and take it out of calibration mode.

During normal operation, the upstream active splitter unit continues tomonitor receiver currents for the incoming upstream links. If there issignificant deviation, it may still issue a non-calibration modedownstream command to re-adjust power, and it can also signal plantissues upstream.

The active splitter units operated in the disclosed manner can alsobuild a map of connected active splitter units. Also, a map can becreated of upstream power from connected ONUs and statistics onindividual ONU operation and link loss can be collected, for instance tolocate chattering ONUs or poor ONU connections.

The head end transmitter can also send a command to downstream activesplitter units to initiate calibration or change a mode of operation(for instance from 200 MHz to 1200 MHz optimized operation). Any othertype of bidirectional EMS system monitoring can be envisioned for activesplitter units that can receive and transmit low data rate traffic. Itshould be understood that this does not require complex or costly HFCEMS systems; minor optical power fluctuations by either pump powervariation or low level signal injection in the downstream signal path,or reverse laser power variation in the upstream path, are sufficient todetect binary or kHz range (like remote control chips) modulated datapatterns. It should also be understood that the most expensiveoption—injection of a downstream optical signal—is only relevant at thehead end, or in the upstream path typically only relevant in 1 out of 33active splitter locations.

Another important consideration is that the CMTS should set up modemlevels correctly. In regular return or RFoG systems, there isconsiderable uncertainty in system levels due to RF components orapplied combiner networks. In the active splitter system, however, thereare no RF components in the link, the service group is aggregated in theoptical domain, and only one low gain, low performance, and low outputlevel receiver is required which is coupled directly to the CMTS returnport. In some embodiments, it may be desirable to produce a dedicatedactive splitter receiver with an accurately calibrated output level as afunction of input modulation index. Such a receiver has no need for awide input range; −3 to +3 (or 0 to +6) dBm is sufficient. The highinput level implies that the gain can be low. The absence of RFcombining following the receiver also means that the output level can below. Therefore, such a receiver should be obtainable in a high density,low power form factor. With such a receiver, little if any RF wiring maybe required in the head end, and the CMTS can accurately set reverselevels to obtain the correct optical modulation index. In some cases,there may be a need to connect other equipment than the CMTS to thereverse path. The receiver may use an auxiliary output to provide forthis functionality, rather than the main output with external RFsplitters. This eliminates any level uncertainty due to RF componentsbetween the receiver and the CMTS.

Embodiments

Some embodiments of the foregoing disclosure may encompass multiplecascaded active splitters that are configured to work with ONUs basedprimarily on optical input levels without requiring bidirectionalcommunication. Other embodiments may encompass multiple cascaded activesplitters that are configured to work with ONUs by using bidirectionalcommunication.

Some embodiments of the foregoing disclosure may include an activesplitter with multiple optical inputs, each providing an optical inputto one or more detectors that together output a combined signal to ahigh pass filter that presents a low impedance to the detectors andrejects all signals below an RF frequency band and passes all signalsabove an RF frequency band before presenting the combined signal to anamplifier and a re-transmitting laser.

Some embodiments of the foregoing disclosure may include an activesplitter with multiple optical inputs, each providing an optical inputto one or more detectors that together output a combined signal, wherethe active splitter has a bias circuit with a sufficiently low impedanceat low frequency such that all detectors can be illuminated at the sametime without a significant drop in bias to the detectors.

Some embodiments of the foregoing disclosure may include an activesplitter with a reverse laser where the reverse laser turns on when aphotocurrent at the active splitter input detectors is above athreshold, and where the slew rate when the laser turns on is limitedsuch that it does not create a transient having a spectrum thatinterferes with the upstream spectrum to be transmitted.

Some embodiments of the foregoing disclosure may include an RFoG activesplitter architecture where reverse lasers of the active splitter(s)and/or ONUs connected to the active splitter(s) are operated with acontinuous output. Some embodiments of the foregoing disclosure mayinclude an RFoG active splitter architecture where reverse lasers of theactive splitter(s) and/or ONUs connected to the active splitter(s) areoperated between a high and a low power mode such that the output poweris high during bursts of upstream transmission and is otherwise low inoutput. Some embodiments of the foregoing disclosure may include an RFoGactive splitter architecture where reverse lasers of the activesplitter(s) and/or ONUs connected to the active splitter(s) may beselectively set to either one of a continuous mode and a burst mode.

Some embodiments of the foregoing disclosure may include an RFoG ONUthat switches between a high and a low output power state where theoutput power is high during burst transmission of information and wherethe low output power state is above the laser threshold.

Some embodiments of the foregoing disclosure may include an RFoG systemthat measures detector currents at all inputs, building a table ofdetector currents during high and low (or no) input power to the opticalinputs and computes, based on that table, a gain value such that amodulation index of the reverse transmitting laser has a known relationto a modulation index at the optical inputs to the active splitter, suchthat the reverse transmitting laser has an optimal modulation index butclipping is prevented, even for the port with the highest optical input.In some embodiments of the foregoing disclosure, the optimal modulationindex of the reverse transmitter is nominally the same as that for theoptical inputs.

Some embodiments of the foregoing disclosure may include an RFoG ONUwith an RF signal detector that detects bursts of input signals andactivates a laser at a high power mode when a burst is detected andotherwise activates the laser at a low power mode, such as zero power.An electrical attenuator may precede the laser driver and may attenuatean RF input signal, such that in the low output power state the lasercannot be clipped by an RF input signal. The RF attenuation before thelaser may be reduced as the laser power increases from the low powerstate, such that the RF attenuation is rapidly removed to have minimalimpact on the burst but during the transition, the laser still is notclipped.

Some embodiments of the foregoing disclosure may include an RFoG ONUwith an RF signal detector that detects bursts of input signals andincludes an electrical attenuator that precedes the laser driver toattenuate the RF input signal, such that when no nominal input ispresent noise funneling by the ONU of weak noise signals into the ONU isprevented and RF attenuation is rapidly removed when a burst is detectedto have minimal impact on the burst.

Some embodiments of the foregoing disclosure may include an RFoG ONUthat can receive a downstream signal instructing it to adjust outputpower level, RF gain or both. In some embodiments, such an ONU canreceive assigned port numbers and status monitoring requests. In someembodiments, such an ONU can transmit upstream information such asstatus, serial number, etc.

Some embodiments of the foregoing disclosure may include an activesplitter that can transmit a downstream signal with requests todownstream units to adjust optical power level, gain or to requeststatus information. Some embodiments may include an active splitter thatcan receive such downstream signals. Some embodiments may include anactive splitter that can transmit and/or receive such signals in theupstream direction, as well.

Some embodiments of the foregoing disclosure may include an ONU with anRF detector, an attenuator, a bias circuit, and a microcontroller wherethe microcontroller estimates laser clipping based on measured RF powerlevels and tracks what fraction of the time the laser is clipping andincreases attenuation in case this fraction exceeds a threshold. Themicrocontroller may also adjust laser bias to prevent clipping. In someembodiments, the microcontroller brings attenuation to a nominal valuewhen RF power to the laser is at or below a nominal value. In someembodiments, changes in attenuation made by the microcontroller takeplace in discrete steps in time and magnitude.

In some embodiments of the foregoing disclosure the microcontroller mayset the attenuation to a high enough level to prevent clipping but lessthan needed to obtain a nominal modulation index.

Some embodiments of the foregoing disclosure may include a bidirectionalRF-over-fiber architecture with more than one re-transmission link inthe reverse direction, where detected signals from preceding links arecombined at each re-transmission link.

Some embodiments of the foregoing disclosure may include a calibratedreceiver at a head-end that provides a specific RF output level for aninput modulation index, with a gain control such that for differentoptical input levels, the RF output level for a given modulation indexis held constant. In some embodiments, a receiver may include twooutputs, at least one connected to a CMTS without any RF combining andsplitting networks.

Some embodiments of the foregoing disclosure may include an activesplitter with at least two gain settings, one gain setting optimized forONUs that can transmit the full reverse spectrum that the system cansupport, and one setting optimized for ONUs that can transmit an amountof spectrum less than the full spectrum that the system can support,where the active splitter combines inputs from multiple ONUs and cantransmit the full spectrum that the system can support.

Some embodiments of the foregoing disclosure may include an activesplitter having adjustable reverse transmission power and adjustablegain such that, for a given received upstream signal modulation index,the active splitter maintains a constant optical modulation indexirrespective of optical output power. In some embodiments, theretransmitted optical modulation index is the same as the receivedoptical modulation index. In some embodiments, the retransmitted opticalmodulation index is a predetermined fraction of the received opticalmodulation index, and the splitter enables an option to vary thatfraction.

Some embodiments of the foregoing disclosure may include an activesplitter that can receive and decode forward communication signals, e.g.an input-monitoring diode for an EDFA, or another monitoring diode.

Some embodiments of the foregoing disclosure may include an activesplitter that can transmit forward communication signals, with forinstance a forward laser, or by modulating the pump current of an EDFA.

Some embodiments of the foregoing disclosure may include an activesplitter that can receive and decode upstream communication signals,e.g. by monitoring upstream detector currents. Some embodiments of theforegoing disclosure may include an active splitter that can transmitupstream communication signals, e.g. by modulating the reverse laser.

Some embodiments of the foregoing disclosure may include a system withat least two active splitters where a first active splitter instructs asecond active splitter to adjust its reverse transmission power level.Some embodiments may use an algorithm to equalize and optimize thereverse transmit level of all downstream active splitters connected toan upstream active splitter. In some embodiments, the algorithm isexecuted automatically at start up such that downstream active splitters(and optionally ONUs) obtain an address and optionally report in theupstream direction the splitter's (or ONU's) serial number and status.In some embodiments, later activation of ports in the splitter leads toan automatic calibration of new ports without interrupting the serviceof existing ports, and with continuous monitoring of port health.

Some embodiments of the foregoing disclosure may include an activesplitter capable of upstream communication, and capable of receiving anddecoding upstream communications from another splitter.

In some embodiments, an active splitter may establish a map of thesystem in which it is included, and may report system status andtopology information to a headend and may issue alarms if necessary. Themap may include serial numbers of active splitters, and may includeserial numbers of connected ONUs. Some embodiments may create a systemmap automatically, and (i) may monitor ONU link input levels to activesplitters; (ii) may detect chattering or otherwise defective ONUs andoptionally instruct active splitter to shut down detectors of defectiveor chattering ONUs; and/or (iii) may monitor the status of the activesplitter that construct the map. In some embodiments, the monitoringfunction is used to automatically trigger route redundancy by monitoringupstream traffic on a link, to determine if the link is intact, and ifthe link is found to be defective, switching downstream traffic to analternate upstream link. In some embodiments, upstream active splittersmonitor downstream active splitters by communicating with downstreamactive splitters.

Some embodiments of the foregoing disclosure may include a head end thatinstructs downstream active splitters to initiate a self-calibrationprocedure.

Some embodiments include a combiner that can monitor each of theupstream input ports and thus detect a loss of a link to such a port.The loss of an upstream link implies that the associated downstream linkhas been lost. Detection of a link can be used to initiate switchingover to a redundant fiber link, preferably following a different fiberroute.

The terms and expressions that have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention, in the use of such terms andexpressions, of excluding equivalents of the features shown anddescribed or portions thereof, it being recognized that the scope of theclaimed subject matter is defined and limited only by the claims thatfollow.

The invention claimed is:
 1. An active splitter comprising: a unitoperating as a splitter in a forward direction and an active combiner ina reverse direction, the combiner having a plurality of inputs that eachreceive an upstream optical signal from a respective optical networkunit (ONU) and combines them to create a combined electrical signal atan output; a transmitter that receives the combined electrical signaland converts the combined signal to a reverse path optical signal, thetransmitter adjusting at least one of power output and gain to maintaina constant ratio between optical modulation index at the output andoptical modulation index at an input, wherein the active splitter has afirst gain setting optimized for optical network units (ONUs)transmitting a first reverse spectrum and a second gain settingoptimized for ONUs transmitting a reverse spectrum limited to an amountless than the first reverse spectrum, wherein the active splitter isconfigured for transmitting the first reverse spectrum; and amultiplexer that multiplexes the reverse path optical signal with aforward path optical signal.
 2. The active splitter of claim 1 where theconstant ratio of optical modulation index at the output is maintainedirrespective of the respective optical power received at the pluralityof inputs.
 3. The active splitter of claim 2 where the opticalmodulation index at the output is set to the optical modulation index ofthe plurality of inputs.
 4. The active splitter of claim 2 where theoptical modulation index at the output is set to an adjustable fractionof the optical modulation index of the plurality of inputs.
 5. A systemcomprising a head end, a plurality of optical network units (ONUs), andat least one active splitter, the active splitter comprising: a unitoperating as a splitter in a forward direction and an active combiner ina reverse direction, the combiner having a plurality of inputs that eachreceive an upstream optical signal from a respective optical networkunit (ONU) and combines them to create a combined electrical signal atan output; a transmitter that receives the combined electrical signaland converts it to a reverse path optical signal at a calibrated level;a multiplexer that multiplexes the reverse path optical signal with aforward path optical signal; where the active splinter at least one of:(i) calibrates ay least one of its own reverse transmission opticalpower level and RF signal gain to that of at least one other device, andin response to a downstream signal; (ii) calibrates at least one of itsown reverse transmission optical power level and RF signal gain to thatof at least one other device, and in response to an upstream signal; or(iii) calibrates at least one of a reverse transmission optical powerlevel and RF signal gain of another device using a downstream signalsent from the active splitter to the other device.
 6. The system ofclaim 5 where the active splitter is capable of decoding receivedforward path communication signals.
 7. The system of claim 5 where theactive splitter is capable of receiving a forward path communicationsignal, modulating the received forward path communication signal, andoutputting the modulated forward path signal to another device.
 8. Thesystem of claim 5 where the active splitter is capable of instructing asecond active splitter to adjust its reverse transmission power level.9. The system of claim 5 where the active splitter constructs a systemmap comprising topology information, the topology information includingat least one of: (i) serial numbers of other active splittersoperatively connected to the active splitter; (ii) serial numbers ofONUs operatively connected to the active splitter.
 10. The system ofclaim 9 where said active splitter is capable of using the map to reportsystem status information and said topology information to a head end.11. The system of claim 10 where said system status information includesthe status of said active splitter.
 12. The system of claim 10 where anupstream active splitter communicates with and monitors a downstreamactive splitter.
 13. The system of claim 9 where said active splitter iscapable of detecting at least one of an input level to an activesplitter from an ONU, and chattering ONUs.
 14. The system of claim 13capable of disabling detectors of at least one of defective ONUs andchattering ONUs.
 15. The system of claim 9 where the active splitter iscapable of automatically triggering route redundancy by monitoringupstream transmission over a first link and switching upstream trafficto a second link if the first link is non-operative.
 16. The system ofclaim 5 where the head end is capable of instructing an active splitterto initiate a self-calibration procedure.
 17. The system of claim 5where the reverse transmit level of all other active splitters connectedto and downstream from said active splitter are equalized to anoptimized level.
 18. The system of claim 17 where said reverse transmitlevel of any downstream active splitter is equalized to said optimizedlevel automatically at the startup of said downstream splitter.
 19. Thesystem of claim 5 where the active splitter is capable of receiving anddecoding an upstream signal, and communicating with an upstream deviceusing the decoded upstream signal.
 20. The system of claim 5 wherecalibration occurs automatically.
 21. The system of claim 5, where atleast one optical network unit (ONU) is capable of adjusting at leastone of output power level or gain, in response to an instructionreceived by said ONU in a downstream signal.
 22. The system of claim 5,where at least one ONU can receive an assigned port number.
 23. Thesystem of claim 5, where at least one ONU can respond to a statusmonitoring request.
 24. The system of claim 5, where at least one ONUcan send its serial number and/or status information to an upstreamdevice.
 25. The system of claim 5 where the head end includes acalibrated receiver that provides a constant RF output level based on amodulation index of an input signal, by adjusting a gain on the RFoutput level as a function of optical input level.
 26. The system ofclaim 25 where the calibrated receiver has two RF outputs, at least onedirectly connected to a CMTS that lacks any RF combining network andlacks any RF splitting network.
 27. An active splitter comprising: aunit operating as a splitter in a forward direction and an activecombiner in a reverse direction, the combiner having a plurality ofinputs that each receive an upstream optical signal from a respectiveoptical network unit (ONU) and combines them to create a combinedelectrical signal at an output; a transmitter that receives the combinedelectrical signal and converts the combined signal to a reverse pathoptical signal, the transmitter adjusting at least one of power outputand gain to maintain a constant ratio between optical modulation indexat the output and optical modulation index at an input, wherein theconstant ratio of optical modulation index at the output is maintainedirrespective of the respective optical power received at the pluralityof inputs; and a multiplexer that multiplexes the reverse path opticalsignal with a forward path optical signal.
 28. The active splitter ofclaim 27 having a first gain setting optimized for ONUs that cantransmit a first reverse spectrum and a second gain setting optimizedfor ONUs that transmit a reverse spectrum limited to an amount less thanthe first reverse spectrum, and where the active splitter can transmitthe first reverse spectrum.
 29. The active splitter of claim 28 wherethe optical modulation index at the output is set to the opticalmodulation index of the plurality of inputs.
 30. The active splitter ofclaim 28 where the optical modulation index at the output is set to anadjustable fraction of the optical modulation index of the plurality ofinputs.